1. Field of the Invention
This invention is directed to methods for enhancing the adhesion of composite electrodes onto conductive plastic foils useful as current collectors, to the foils prepared by these methods and to electrochemical cells produced from these foils.
2. State of the Art
Electrochemical cells comprise a cathode, an anode and an electrolyte interposed therebetween. The electrochemical cells are often defined as liquid or solid cells and this refers merely to whether the electrolyte interposed between the anode and the cathode is a liquid or a solid. Solid electrochemical cells are well known in the art and present many advantages over conventional liquid batteries such as improved safety features, lighter weight, etc.
In order to enhance the overall current produced by solid or liquid batteries, it is conventional to employ several electrochemical cells in a battery. When so employed, the current from each of the cells is accumulated so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrochemical cells employed in the battery.
One method for accumulating the current from individual electrochemical cells is by using a current collector attached to the cathode or the anode of the electrochemical cell. Typically, the current collector is a metal foil or a conductive plastic which is coupled to other current collectors in the battery so that the current generated by each cell is collected and accumulated over all of the cells. Thus, the total current generated by the battery is a summation of the current generated by each of the electrochemical cells employed in the battery minus whatever current is lost due to resistance in the current collector.
Notwithstanding the benefits of using current collectors in electrochemical cells, metal foil current collectors are heavier than conductive plastic foil current collectors and adversely increase the weight of the battery. On the other hand, when conductive plastic foils are used as the current collector, they are in direct contact with a composite electrode and are suspectable to swelling due to contact of the current collector with the electrolytic solvent found in the electrode. Such swelling adversely affects the performance of the battery.
Additionally, some conductive plastic foil current collectors possess a relatively non-adherent surface which retards the attachment of a composite electrode to the surface of the current collector. Specifically, composite electrodes are typically prepared from a paste which is applied onto the surface of the current collector by conventional means such as extrusion and then cured to form a solid composite electrode. When the surface of the current collector is non-adherent, then during application of a paste onto the foil, the paste can become dislodged from the foil. In turn, after curing, such dislodgement will result in defects in collecting current from that cell.
This invention is directed to the discovery that the inclusion of a layer of electrically-conducting adhesion-promoter to the surface of conductive plastic foil current collector provides a means to both increase the adherence of the electrode to the foil and to retard contact of the electrolytic solvent with the plastic foil.
Thus, while the art discloses that polymers containing grafted acid functionalities (i.e., maleic acid or anhydride) have improved adherence (see U.S. Pat. Nos. 4,810,755 and 4,857,600 as well as International Patent Application Publication No. WO 91/02760), there is nothing in the art which discloses the use of a layer of an electrically-conductive adhesion promoter to the surface of the plastic foil.